The overall process in which lignin is extracted from wood chips to produce cellulose in pulp manufacture is well developed. An aqueous solution of two kinds of active alkali, such as sodium hydroxide and sodium sulfide, forms a white liquor in which wood chips, or other cellulosic materials, are digested for several hours under heat and pressure in the kraft pulping process.
In the digesting process, the lignin is dissolved by the white liquor and is removed from the cellulose fibers of the wood. At this point the organic lignin fraction becomes a part of the 15% solid content of the aqueous solution which is now termed weak black liquor.
Losses of sodium and sulfur compounds normally occur in the pulp and flue gas streams leaving a pulp mill. Therefore, amounts of sodium sulfate are added to the heavy black liquor as makeup. A substantial degree of concentration of the weak black liquor is carried out in direct and/or indirect evaporators, producing a heavy black liquor with a solid content of 63% to 68% when normal North American hard and soft woods are pulped. The pulping of tropical hardwoods, or non-wood materials like bamboo, bagasse, or straw, by contract, produces highly viscous liquors which cannot be concentrated above about 40-50% solids. These require the burning of large amounts of expensive auxiliary fuels (gas or oil) to sustain combustion in a boiler. This practice is unsatisfactory, also, because of low reduction efficiency, high emission of sulfur dioxide, and a greatly increased chance of blackouts and smelt-low solids liquor (steam) explosions.
It is conventional practice to introduce a coarse spray of heavy black liquor at an elevation mechanically well above the hearth in a chemical recovery boiler. This spray falls through the hot gases rising from the burning char bed on the hearth. During the descent of the drops through the hot gases, the remaining 32-37% water is evaporated and the dry solid particles fall to the hearth at the bottom of the boiler where they are pyrolized. Pyrolysis produces char which is burned, leaving the inorganics as molten smelt. This smelt is dissolved in water to form an aqueous solution called green liquor. Causticizing the green liquor by adding slaked lime completes its reconstitution into white liquor by changing inactive sodium carbonate into active sodium hydroxide so that it may be brought into contact with wood chips in the digestion to continue with the pulp-making process.
It is desirable that as much of the water of the black liquor, as commercially feasible, be driven from the solids before pyrolysis, burning, and the formation of the smelt is attempted in the present boiler. Under the present practice, this automatic sequential treatment of the black liquor requires a boiler volume large enough to accommodate the complete sequence of black liquor treatment. Evaporation of the approximate 32-37% residual water of the black liquor in the furnace cavity cools and slows the burning process.
The operation of the boiler is somewhat complex. Mechanically, the black liquor is spray-formed into about 1/4" to 1/2"-size drops at about 15 feet above the hearth. The drops cannot be smaller than about 1/4", or some of the molten smelt, formed by burning the small liquor particles in flight, will be swept up by the combustion air to foul boiler heat transfer surfaces.
If the black liquor solids can be formed in a coat on a porous extended surface, and the water can be evaporated from the black liquor solids faster and more completely before introduction into the boiler, the resulting charge of dry black liquor solids will enable a significant decrease in the size of the boiler and increase in the bed-burning temperature and reduction efficiency. In addition, if the extended surface is provided by porous combustible material, the combustible material will become an additional source of heat for the boiler. In short, there is a need to shift the final water evaporating step upstream of the boiler to obtain significant improvements in economy and speed in forming smelt, with essential active alkali, and steam from the black liquor solids. No present commercially feasible process is available to permit this important shift.